1) Field of the Invention
The present invention relates to an optical transmission system in which optical amplification is realized in a plurality of gain bands. The present invention also relates to an optical terminal used in the above optical transmission system. The present invention further relates to an optical repeater used in the above optical transmission system.
2) Description of the Related Art
Recently, optical fiber amplifiers which utilize a nonlinear optical phenomenon occurring in optical fibers and being called Raman amplification are receiving attention in the field of optical communications. In the Raman amplification, a physical phenomenon in which vibrations of materials inelastically scatter incident light so as to produce Raman scattered light having a wavelength which is different from the wavelength of the incident light is utilized, and strong excitation light is injected into an optical-fiber transmission line so that optical amplification occurs in the entire optical-fiber transmission line. For example, excitation light having a wavelength of 1.45 micrometers is injected into the optical-fiber transmission line in order to amplify an optical signal having a wavelength of 1.55 micrometers.
When repeaters are arranged to realize the Raman amplification, a longer optical fiber cable can be laid, and intervals between broadband repeaters can be increased.
Usually, excitation light of more than one wavelength is used for realizing Raman amplification in more than one gain band corresponding to the more than one wavelength, where the excitation light having more than one wavelength is generated by more than one light source having predetermined output power. When the number of optical signals transmitted in each gain band is identical, an identical gain is achieved in each gain band, and satisfactory transmission characteristics are obtained.
However, when the number of optical signals transmitted in one gain band is smaller than the number (or numbers) of the other gain band (or gain bands), the power of excitation light corresponding to the gain band in which the smaller number of optical signals are transmitted is intensively converted into the optical signals in the smaller number. Therefore, gains in the respective gain bands differ, and thus transmission characteristics deteriorate.